1. Field of the Invention
The present invention relates to a dot printing method for a dot printer which prints characters, marks or graphic patterns by using dot element such as mechanical type, electrical type or chemical type, and particularly to a dot printing method for executing high-quality printing.
2. Description of Related Art
Conventionally, dot printers, such as impact type, electrostatic type, thermal type and inkjet type are well known. A method for executing high-quality printing using the above-mentioned dot printers is disclosed in Japanese Patent Publication No. 56-6032. According to the print method, a printing operation is executed twice in order to print characters, marks or graph patterns corresponding to one line. That is, in order to print odd dots in a column direction with dot elements of a print head, a first printing operation is executed while moving the print head in a row direction. Next, the print head and a sheet are relatively displaced by a small amount in the column direction. Finally, a second printing operation is executed while moving the print head in the row direction in order to print even dots in the column direction, that is at positions between the printed odd dots. The result is high-quality printing because the dots are printed close together producing a darker image.
In the dot printing method, there are two types of print method to execute two printing operations in the row direction. A first type of print method is as follows. First, during the displacement of the print head from the left side to the right side of the row or printing line, the first printing operation is executed. Next, when the print head is returned from the right side to the left side of the row, the second printing operation is executed. Using this first method, high-speed printing can be carried out. However, shifts or deviations in printing positions between the odd dots and the even dots in the column direction result due to problems such as backlash in the mechanism for displacing the print head in the row direction and low accuracy in the stop positions of the motor driving the mechanism. Consequently, the quality of dot printing is reduced.
A second type of the print method is as follows. First, when the print head is displaced from the left side to the right side of the row or printing line, the first printing operation is executed. Next, the print head is returned from the right side to the left side without executing a printing operation. Finally, the print head is again displaced from the left side to the right side as the second printing operation is executed. According to this second method, high-quality printing can be carried out. However, in order to execute two printing operations, the print head has to be displaced three times along the row or printing line, thereby reducing the dot printing speed.